Application of multipath combiner and equalizer in a multi-channel direct sequence spread spectrum communication system

ABSTRACT

A receiver subsystem in a multi-channel direct sequence spread spectrum system consists of a signal register, a multipath detector, a multipath combiner, a decision-feedback equalizer, a multi-channel detector, and a multi-channel signal regenerator. First, the multipath detector finds out the information about various components of a multipath-fading signal. Then the multipath combiner combines the significant components from corresponding tapped outputs of the signal register together. Next the decision-feedback equalizer eliminates interference from the output signal of the multipath combiner. After receiving signal from the decision-feedback equalizer, a multi-channel detection circuit recovers the information and estimates the complex signal for each channel. The multi-channel signal regenerator resorts to these complex signals to generate the estimated transmission signal. It multiplies each estimated complex signal by a corresponding spreading signal, summarize all the products together, and then multiply the summation by a scrambled signal. Finally, the regenerated multi-channel signal and the delayed version of the output signal of the decision feedback equalizer are compared to produce error signal for updating the coefficients of the decision feedback equalizer.

FEDERALLY SPONSORED RESEARCH

[0001] Not Applicable

SEQUENCE LISTING OR PROGRAM

[0002] Not Applicable

FIELD OF THE INVENTION

[0003] The invention is generally related to a receiver in a communication system. More particularly, the invention is related to applying both multipath combiner and decision-feedback equalizer to a receiver in a multi-channel direct sequence spreading spectrum communication system for utilizing and combating the fading over several symbol periods.

BACKGROUND OF THE INVENTION

[0004] In a wireless communication system, especially in a mobile communication system, fading occurs from times to times. Buildings, mountains, and foliage on a transmission path can cause reflection, diffraction, and scattering on a propagating electromagnetic wave. The electromagnetic waves reflected from various large objects, travel along different paths of varying lengths. If an obstacle on the transmission path has sharp irregularities, the secondary waves resulting from the obstructing surface are present around the obstacle. Also if there are small objects, rough surfaces, and other irregularities on the transmission path, scattered waves are created. All these waves interact with each other and cause multipath fading at specific locations. The multipath fading can seriously deteriorate the quality of a communication system.

[0005] Usually there are two methods to deal with the problem caused by multipath fading. One method is to use a multipath combiner such as a RAKE receiver to combine the significant paths of a multipath-fading signal together. Another method is to use an equalizer to eliminate all paths of a multipath-fading signal except the strongest path.

[0006] A multipath combiner in a receiver is to combine significant paths together. It works effectively when the most significant paths have almost same strength and these paths do not nullify. Looking at these paths on time domain, one can see that a transmitted symbol appears several times with each time corresponding to a different path. When transmission rate is low, the multipath fading spans in less than one symbol period. Through the multipath components interfere with each other, the transmitted symbols do not cancel each other totally. However, as the transmission rate is higher and higher, a multipath fading could easily span one or more symbol periods and a symbol on one path could be almost cancelled by a previous symbol on other path. When symbols almost cancel each other or possess nulls, a multipath combiner has nothing to combine and therefore system performance declines.

[0007] A decision-feedback equalizer is effective to compensate the nulls. Basically, an equalizer in a receiver is to keep the strongest path and eliminate all other paths. It works effectively when there is a strong and stable path.

[0008] However, when a multipath fading spans one or more symbol periods and no any path is stable, either multipath combiner or equalizer does not work effectively. One example for this kind of scenario is a high-speed mobile receiver in metropolitan area. In order to have reliable communication under this kind of scenario, one may want to make the adjacent transmitted symbols different and utilize a multipath combiner to combine all these significant paths together.

[0009] A direct sequence spreading spectrum system is able to make adjacent symbols different. The delayed versions of the transmitted pseudo-noise (PN) signal have poor correlation with the original PN signal and therefore a direct sequence spread spectrum system is multipath resistant. A multipath fading causes PN chips interfered with each other. The interference caused by one chip to other chips is called interchip interference. For detection is based on a group of PN chips, or on symbol, the mess caused by multipath fading is less severe. Usually chip rate is much higher than symbol rate and a PN period is much longer than a symbol period, therefore it is possible to distinguish the components of a multipath fading signal spanned over several symbol periods. Together with a multipath combiner, a direct sequence spread spectrum system can combine the information obtained from several resolvable multipath components and therefore improve the system performance.

[0010] One disadvantage of a direct sequence spreading spectrum system is low spectral efficiency. In order to transmit data with higher rate, a multi-channel direct sequence spreading spectrum communication system is introduced.

[0011] In a multi-channel communication system, the multipath fading could be more serious. First, each component of a multipath-fading signal on a particular channel only not interferes with the other components of the signal on the same channel, but also interferes with each component of the signals on other channels. The interference caused by a component on a channel to other components on the same channel is called intersymbol interference. The interference caused by a component on a channel to the components on other channel is called interchannel interference. Second, even if each symbol on each channel has poor correlation with their neighbor symbols, a delay version of the current symbol on a particular channel could almost cancel some future symbol on one of the other channels and possess null on that channel.

[0012] Based on above discussion, it is interesting to employ both multipath combiner and decision-feedback equalizer in a multi-channel communication system.

[0013] One way to employ both multipath combiner and equalizer in a receiver of multichannel communication system is to use an equalizer and a multipath combiner for each channel. There are two problems. One problem is huge hardware consumption. For a multi-channel system with 64 channels, there will be 64 multipath combiners and 64 equalizers. Another problem is that each equalizer works independently, which may not be a good choice to subdue interchannel interference. In order to reduce interchannel interference, one may introduce some connections among these equalizers, which makes the design even further complex.

[0014] Another way to employ both multipath combiner and equalizer in a receiver of multi-channel communication system is to use an equalizer for selecting each significant path of a multipath fading signal and to use a multipath combiner to combine the signals from all of these equalizers together. There are some problems also. The several significant paths do not have equal power. The equalizer, which selecting a path with relatively small signal strength, may not work effectively especially when the path is not stable. Also, in metropolitan area, usually at least 4 paths have to be considered and therefore there must be at least 4 equalizers. The hardware consumption is still a lot.

[0015] Therefore, in order to save hardware, it is attractive to use one multipath combiner to combine the signals of several significant paths together and use one decision-feedback equalizer to take care of interference in a multi-channel communication system.

OBJECTIVES OF THE INVENTION

[0016] The primary objective of the invention is to utilize and combat the multipath fading in a multi-channel direct sequence spread spectrum communication system by employing a multipath combiner to combine the signals from several significant paths together and an equalizer to eliminate the interchip interference, intersymbol interference, and interchannel interference.

[0017] Another objective of the invention is to utilize and combat the multipath fading in a direct sequence spread spectrum communication system by employing a multipath combiner to combine the signals from several significant paths together and an equalizer to eliminate the interchip interference and intersymbol interference.

[0018] Another objective of the invention is to utilize and combat the multipath fading in a communication system by employing a multipath combiner to combine the signals from several significant paths together and an equalizer to eliminate the intersymbol interference.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The accompanying drawings, which are incorporated in and constitute a part of the specification, depict the preferred embodiment of the present invention, and together with the description, serve to explain the principle of the invention. In the figures, like reference numerals refer to the same or similar elements.

[0020]FIG. 1 illustrates spreading signals for each of multi-channels and a scrambled signal of a multi-channel direct sequence spread spectrum communication system.

[0021]FIG. 2 illustrates the essential of a transmitter in a multi-channel direct sequence spread spectrum communication system with the spreading signals and the scrambled signal as in FIG. 1.

[0022]FIG. 3 illustrates the essential of a receiver with a multipath combiner and a decision feedback equalizer in a multi-channel direct sequence spread spectrum communication system with the spreading signals and the scrambled signal as in FIG. 1.

[0023]FIG. 4 illustrates a signal regeneration device for a multi-channel direct sequence spread spectrum communication system with the spreading signals and the scrambled signal as in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0024] Detailed description of the preferred embodiment is provided herein. The embodiment illustrates the essential of a receiver with a multipath combiner and a decision feedback equalizer in a multi-channel direct sequence spread spectrum communication system by way of example, not by way of limitation. It is to be understood that it could be easy for those skilled in the art to modify the embodiment in many different ways. For example, when there is just one channel in a multi-channel direct sequence spread spectrum communication system, then the system will become a regular direct sequence spread spectrum communication system. Therefore, specific details disclosed are not to be interpreted as limitations, but rather as bases for the claims and as representative bases for teaching one to employ the present invention in virtually any appropriately detailed system, structure or manner.

[0025]FIG. 1 shows spreading signals for each of multi-channels and a scrambled signal of a multi-channel direct sequence spread spectrum communication system.

[0026] There are N spreading signals 105 ₁ to 105 _(N) with one for each of the N channels. These N spreading signals S₁ to S_(N) are different from each other. Actually, they are orthogonal in many cases. Also in many systems, the spreading signals S₁ to S_(N) do not change from one symbol period to another.

[0027] There is a scrambled signal PN 110. The section of PN signal in the first symbol period is denoted by PN₁, in the second symbol period is denoted by PN₂, and so on. Assume L is the number that no significant component of a multipath signal will span over more than L symbol periods. In order to distinguish multipath components of a signal, the section of a scramble signal in a symbol period is different from the section in any of its 2L neighbor symbol periods.

[0028]FIG. 2 shows the essential of a transmitter corresponding to the signals in FIG. 1.

[0029] There are N quadrature amplitude modulation (QAM) mapping devices 205 ₁ to 205 _(N). The bit stream for each channel is divided into a series of blocks with each block having a predetermined number of bits. Each of the mapping devices 205 ₁ to 205 _(N) maps a corresponding series of blocks into a complex signal, which consists of a series of complex numbers.

[0030] There are N multipliers 210 ₁ to 210 _(N). Each multiplier multiplies a complex signal with a corresponding spreading signal from the set of spreading signals S₁ to S_(N). The adder 215 sums up the products from all these multipliers together. The multiplier 220 further multiplies the summation by a scrambled signal PN.

[0031]FIG. 3 shows the essential of a receiver with a multipath combiner and a decisionfeedback equalizer in a multi-channel direct sequence spread spectrum communication system with the spreading signals and the scrambled signal as in FIG. 1. The receiver is corresponding to the transmitter in FIG. 2.

[0032] The signal register 305 consists of a plurality of registers with the first register connected to input signal S_(1n) and each of the rest registers connected to its previous one. Usually a sampling clock drives all these registers. There are M tapped output signals. These M tapped output signals are a moving section of the down-sampled signal of the input signal S_(1n). The delay between any two adjacent output signals is the same. When there is a tapped output every K registers, then the down-sampling rate is K. When there is a tapped output each register, there is no down sampling and K is equal to 1.

[0033] The multipath detection device 310 detects multipath information from the output signals of the signal register 305. A multipath detection device could be built based on matched filter or correlator.

[0034] The controller 315 extracts information about each path of a multipath-fading signal from the output of multipath detection device 310. The controller 315 will also generate various control signals such as path selecting signals, path strength signals, and path timing signals.

[0035] The multipath combiner basically consists of a switch box and a signal combiner.

[0036] The switch box is a plurality of gate circuits. With the control signals from controller 315, the switch box 320 selects several taped outputs from the signal register 305. These taped outputs correspond to the several significant paths. The maximum number of selected taped outputs is predetermined.

[0037] The signal combiner 325 combines the selected tapped output signals together. Usually there are three combining methods, that is, maximum ratio combining (MRC), equal gain combining (EGC) and selection combining (SLC). A signal combiner could consist of delay circuit, phase rotating circuit, weighting circuit, and summation circuit. The signal combiner 325 has different structure for different combining method.

[0038] For maximum ratio combining, the delay circuit inserts different delay for different path so that these paths can align up on time. The phase rotating circuit rotates each path by some degree so that all these paths have a same phase. The weighting circuit assigns each path a weight corresponding to the signal strength of that path. Finally, the summation circuit combines all the paths together according to their weights. The output signal from summation circuit is a combination signal, which is statistically, a better estimate of the transmitted signal than each of these component signals is.

[0039] The output of the signal combiner 325 only has one significant component instead of several significant components. With only one significant component in its input signal, an equalizer works effectively.

[0040] Proper delaying and down-sampling could be inserted so that the signal combiner 325 has chip rate output signal whose significant component aligned up with a symbol clock.

[0041] A decision feedback equalizer takes its input signal from the output of the signal combiner 325. The decision feedback equalizer consists of a feedforward filter 330, a feedback filter 335, a summation circuit 340, a detection circuit 345, and error signal generator 365.

[0042] The feedforward filter 330 is a linear transversal filter. A linear transversal filter is made up of tapped delay lines, with the tappings spaced at the chip interval T_(c). The feedforward filter 330 takes the output signal from signal combiner 325 as its input. Functionally, it is to remove the interference caused by future symbols from the estimates of the current symbols.

[0043] The feedback filter 335 is also a linear transversal filter with tappings spaced at the chip interval T_(c). It takes the decisions on previously symbols as its input. Functionally, the feedback filter 335 is to remove the interference caused by previous detected symbols from the estimates of the current symbols.

[0044] The summation circuit 340 adds up the output signals from both filters.

[0045] The multi-channel detection device 345 takes its input from the output of summation 340. It could be built based on matched filter or on integrate-and-dump detector. The multi-channel detection device 345 demodulates the information on each channel. The multi-channel detection device 345 has two kinds of outputs. One is a group of complex signals with each one corresponding to a channel. Each complex signal consists of a series of complex numbers with each of these complex number is an estimation of the corresponding complex number at transmitter. Another is a group of blocks of data bits. Each block of data bits is the inverse mapping a corresponding complex number into a predetermined number of data bits.

[0046] The multi-channel signal regeneration circuit 350 receives all the complex signals and generates a multi-channel signal, which is the estimation of the transmitted multi-channel signal. The details of the multi-channel signal regeneration circuit 350 is shown in FIG. 4.

[0047] The delay circuit 355 also takes its input from the output of summation 340. It inserts proper delay so that the signal from delay circuit 355 and the signal from the multi-channel signal regeneration circuit 350 align up on time.

[0048] The subtraction circuit 360, receiving signals from both delay circuit 355 and multi-channel signal regeneration device 350, is to generate a difference signal between its two input signals.

[0049] The error signal generator circuit 365, receiving the difference signal from subtraction circuit 360, produces an error signal which is sent to both feedforward filters 330 and feedback filter 335 to update the coefficients of each filter.

[0050]FIG. 4 shows the signal regeneration device for a multi-channel direct sequence spread spectrum communication system with the spreading signals and the scrambled signal as in FIG. 1. FIG. 4 provides details for the multi-channel signal regeneration circuit 350 in FIG. 3.

[0051] Each of the multipliers 410 ₁ to 410 _(N) multiplies a complex signal with a corresponding spreading signals S₁ to S_(N). The adder 415 sums up all the products together. The multiplier 420 multiplies the summation with a scrambled signal PN. The result signal is a regenerated multi-channel direct sequence spread spectrum signal. 

What is claimed is:
 1. A receiver subsystem in a communication system for utilizing and combating the multipath-fading signal spanned over several symbol periods, comprising: a signal register, receiving a input signal, for generating a plurality of output signals which is a moving section of the sampled signal of the input signal with the delay between any two adjacent output signals being equal; a multipath detection device, which is coupled to signal register, for detecting the information about various components of a multipath signal, a multipath combiner, which is coupled to signal register, for combining significant components; an equalizer, receiving signal from the multipath combiner, for eliminating various interference; a signal regeneration device, which is coupled to the equalizer, for regenerating the estimated transmission signal; and a controller for extracting information on multipath components and generate various control signals.
 2. A receiver subsystem in a communication system for utilizing and combating the multipath-fading signal spanned over several symbol periods as in claim 1, wherein an equalizer is a decision feedback equalizer consisting of a feedforward filter, feedback filter, a summation circuit, a detection circuit, and an error signal generator.
 3. A receiver subsystem in a communication system for utilizing and combating the multipath-fading signal spanned over several symbol periods as in claim 2, wherein a detection circuit comprises an integrate-and-dump detector.
 4. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath-fading signal spanned over several symbol periods, comprising: a signal register, receiving a input signal, for generating a plurality of output signals which is a moving section of the sampled signal of the input signal with the delay between any two adjacent output signals being equal; a multipath detection device, which is coupled to signal register, for detecting the information about various components of a multipath signal; a multipath combiner, which is coupled to signal register, for combining significant components; an equalizer, receiving signal from the multipath combiner, for eliminating various interference; a multi-channel signal regeneration device, which is coupled to the equalizer, for regenerating the estimated multi-channel transmission signal; and a controller for extracting information on multipath components and generate various control signals.
 5. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 4, wherein an equalizer is a decision feedback equalizer consisting of a feedforward filter, feedback filter, a summation circuit, a multichannel detection circuit, and an error signal generator.
 6. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 4, wherein a multipath combiner consists of a switch box and a signal combiner.
 7. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 6, wherein a signal combiner is selected from the group consisting of maximum ratio combiner, equal gain combiner, and selection combiner.
 8. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 4, wherein a multipath detection device comprises an element selected from the group consisting of a set of matched filters and a set of correlators.
 9. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 5, wherein a multi-channel detection circuit comprises a set of matched filters.
 10. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 5, wherein a multi-channel detection circuit comprises a set of correlators.
 11. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 4, wherein a multi-channel signal regeneration device consists of a plurality of multipliers and an adder, with each multiplier multiplying a detected complex signal by the corresponding spreading signal and the adder adding all the products together with the summation further multiplied by a scrambled signal.
 12. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath-fading signal spanned over several symbol periods, comprising a signal register, receiving a input signal, for generating a plurality of output signals which is a moving section of the sampled signal of the input signal with the delay between any two adjacent output signals being equal; a multipath detection device, which is coupled to signal register, for detecting the information about various components of a multipath signal; a multipath combiner, which is coupled to signal register, for combining significant components; a first transversal filter, receiving signal from the multipath combiner, for eliminating the interference from the future chips; a multi-channel detection circuit for detecting the signal on each channel; a multi-channel signal regeneration device, which is coupled to the multi-channel detection circuit, for regenerating the estimated transmission signal; a second transversal filter, receiving signal from multi-channel signal regeneration device, for eliminating the interference from the past chips; and a controller for extracting information on multipath components and generate various control signals.
 13. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath-fading signal spanned over several symbol periods as in claim 12, further comprising: a summation circuit for adding the signals from the first transversal filter and the second transversal filter together; a delay circuit for delaying the signal from the output of the summation circuit for a certain time so that its output signal will be aligned up with the output signal from the multi-channel signal regeneration device on time; a subtraction circuit for finding the difference between the two signals, one from the delay circuit and another from the multi-channel signal regeneration; and an error signal generator, which is coupled to the subtraction circuit, for providing the error signal to both the first transversal filter and the second transversal filter for updating their coefficients.
 14. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath-fading signal spanned over several symbol periods as in claim 12, wherein a multipath combiner is a maximum ratio combiner.
 15. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 12, wherein a multi-channel signal regeneration device consists of a plurality of multipliers and an adder, wherein a multi-channel signal regeneration device consists of a plurality of multipliers and an adder, with each multiplier multiplying a detected complex signal by the corresponding spreading signal and the adder adding all the products together with the summation further multiplied by a scrambled signal.
 16. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 12, wherein a multipath detection device comprises a set of matched filters.
 17. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 12, wherein a multipath detection device comprises a set of correlators.
 18. A receiver subsystem in a multi-channel direct sequence spread spectrum communication system for utilizing and combating the multipath fading signal spanned over several symbol periods as in claim 12, wherein a multi-channel detection circuit comprises an integrate-and-dump detector for each channel. 